1. Field of the Invention
This invention deals with optical modulation, more particularly to a method and apparatus for modulating optical carriers with relatively weak electrical, acoustic, magnetic or similar energy sources.
2. Description of the Related Art
The modulation efficiency of electro-optic optical modulators has a direct impact on the efficiency of optical systems that process radio frequency, microwave, and millimeterwave signals (collectively referred to here as electrical signals). Here, processing of electrical signals refers to, but is not limited to, the following: point-to-point transmission, communications networks, delay or storage, distribution, and phased array beamforming. Most modulators exhibit large switching (on-off) voltages, therefore, small input electrical signals have little effect on the transmitted optical carrier. The modulated output then consists of a strong unmodulated optical carrier with very weak signal sidebands.
In general, the electrical signal power and signal-to-noise ratio carried by an optical carrier is proportional to the optical power. Thus, in most devices, high optical power levels are desirable. However, weakly modulated optical waves (those having small modulation indices) leave significant unmodulated power in the original carrier. Hence, despite a high average optical power needed to increase the electrical signal power, the electrical signal power carried may remain quite small. The excess optical power is detrimental in at least two ways. First, optical amplifiers are limited by the average output power; therefore, amplifiers will have limited use to boost the weakly modulated optical wave. Second, and more importantly, the average optical power must be currently kept below .about.2 mW to a) avoid signal reductions and distortions in high-speed photodetectors and b) to avoid damage to the photodetector. Improvement in photodetector power handling ability (by new design or choice) will extend the power limit; but excess optical power is still expected.